Urine specimens from 65 patients with glomerular disease and from 62 controls were tested for the presence of immune complexes by agarose gel zone electrophoresis. Urinary immune complexes (UICs) were found in 18% of patients, correlating with the simultaneous occurrence of serum immune complexes and depending differentially on the histologic form of glomerular disorder. The UICs were found preponderantly in patients with severe crescentic glomerulonephritis, who had "gaps" in the glomerular basement membrane; in patients with epimembranous electron-dense deposits; and in patients with membrano-proliferative glomerulonephritis. The UICs were detected in no patients with focal glomerular sclerosis or mesangial proliferative glomerulonephritis and in only one of 23 patients with minimal-change disease. None of 62 controls had UICs. Passage of immune complexes through filtering structures of the kidneys may be a consequence of their focal or diffuse damage. Detection of UICs may provide a noninvasive means of assessing the extent of tissue injury in patients with glomerular disorders.